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Old 11-20-2011, 09:44 AM   #1
PrayingMantis
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Problem reducing size for CF Card image using USB


Hi,

I am having an existing CF card image of size 512M, which I wish to unpack to a 4GB USB Stick, upgrade the kernel and initrd on the image and pack it again. So basically here is what I'm doing:

_1. I dd out the existing 512M image to the usb stick, mounted on /dev/sdb
dd if=cf_image.img of=/dev/sdb bs=16384
I observe, the size of the USB is now shown as 512M.
_2. I upgrade the kernel and the initrd on the USB stick.
_3. When I pack the usb back, the size of the image file i get is 4GB and not 512M.
dd if=/dev/zero of=/dev/sdb/zero bs=16384
rm -f zero
dd if=/dev/sdb of=./new_cf.img bs=16384

What am I doing wrong?
Will a sync help?

BTW my img file has two partitions, one small linux partition containing grub to boot up, and a FAT16 partition containing the kernel and the initrd image.
 
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Old 11-21-2011, 12:51 PM   #2
TobiSGD
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Don't copy the image to the USB stick. Just mount it (or better a copy of it, if something goes wrong), make your changes and use dd to copy it back to the CF card.
 
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Old 11-22-2011, 12:44 PM   #3
PrayingMantis
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Hi Tobi,

Thanks for the reply.

But I'm not able to mount the CF card image. The image consists of a small linux partition (containing grub for booting up) and a windows partition (have the kernel and the initrd). It asks for the file system type while mounting.

Mantis
 
Old 11-22-2011, 01:05 PM   #4
TobiSGD
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When it contains partitions you have to mount it with an offset. To get the right offset value just use
Code:
fdisk -l YOURIMAGEFILE
It will show something like this:
Code:
tobi@monster ~ :) % fdisk -l test.img

Disk test.img: 536 MB, 536870912 bytes
255 heads, 63 sectors/track, 65 cylinders, total 1048576 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x7b36c16a

   Device Boot      Start         End      Blocks   Id  System
test.img1            2048       43007       20480   83  Linux
test.img2           43008     1048575      502784   83  Linux
As you can see, the first partition begins with sector 2048 and the sector size is 512 bytes, so the correct offset is 2048*512 = 1048576. To mount the first partition of my image file to /mnt you would launch the mount command this way:
Code:
mount -o loop,offset=1048576 test.img /mnt
In this example the offset for the second partition would be 43008*512 = 22020096.
 
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Old 11-23-2011, 10:22 AM   #5
PrayingMantis
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Hi Tobi,

It works.. !! I have mounted the image file and made changes.

Thank you very much..

Mantis
 
  


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